There is something called cooling drag, where the drag that it takes to cool the engine comes into play. You only need what the engine requires to cool it, the rest is drag. You might find some formulas in the aviation community.
Read through this report;
http://publications.lib.chalmers.se/records/fulltext/244208/244208.pdfBack in my race car days, we often use mesh screen for block offs since it would allow some air through but the majority of it piled up and rolled off the car. At least that's the theory I recall reading somewhere.
